Did you replace the amplifier and sensor tube also? The amplifier should be replaced with the servo.....the tube turns to dust....thus the system can't tell the temperature..... And did you unfreeze the aux water pump? This is what blows the amplifier....and the servo won't work correctly......the center flaps are open when ever the system is on cool.....the close when on heat or deforst......I doubt you have a vacuum leak....as the car wouldn't turn off....

Read all about it here

http://www.imperialclub.com/Repair/Lit/Master/281/cover.htm

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